The present invention relates to an image forming apparatus that forms a transparent toner layer on the outermost surface of a recording material and improves a separating ability of a recording material being passing through a fixing device.
In electro-photographic type image forming apparatus, such as copying machines, printers, facsimile machines and composite machines provided with their various functions, a latent image corresponding to a document is formed on a photoreceptor and visualized by being provided with toner, the visualized toner image is transferred onto a recording paper sheet, thereafter, the toner image transferred onto the recording paper sheet is fixed, and then the fixed recording paper sheet is discharged.
As a fixing device to fix a toner image in such a way, there is a heat roller fixing type fixing device which heats and presses a recording paper sheet, on which a toner image is transferred, while pinching and conveying the recording paper sheet at a nip section formed by a fixing roller incorporating a halogen heater and the like therein and a pressing roller pressing the fixing roller. Since such a fixing device has a simple structure, it has been widely utilized.
Further, there is a belt fixing type fixing device which comprises an endless fixing belt wound around a heating roller incorporating a halogen heater and the like therein and a fixing roller and a pressing roller pressing the fixing roller across the fixing belt, and heats and presses a recording paper sheet, on which a toner image is transferred, while pinching and conveying the recording paper sheet at a nip section formed by the fixing belt and the pressing roller. In such a fixing device, since the heat capacity of the fixing belt is small, there are advantages that a warming-up time can be shortened and energy can be saved.
In the fixing device, when recording paper sheet passes through the nip section, toner of a toner image formed on the recording paper sheet is heated. Accordingly, since the heated toner is provided with adhesive force, there is fear that the recording paper sheet having passed through the nip section adheres to a surface of the fixing roller or the fixing belt, winds around them, and causes jamming. Especially, in the case where a thin sheet paper with a small basic weight or a print-use coated sheet paper is employed, the separating ability becomes low more.
On the other hand, if the diameter of a fixing roller is made large such that a nip width becomes a sufficient length corresponding to the speed-up in an image forming apparatus, since a curvature factor of a roller at the exit of a fixing nip becomes small due to it, the separating ability becomes low.
Further, in the case where a plurality of color toners is superimposed in order to form a color image, since amount of toners increases, the adhesive force of the superimposed toners increases.
Therefore, a separating section is employed in order to separate a recording paper sheet mechanically from a fixing roller and the like. As such a separating section, at a sheet discharging side of a nip section, a separating claw coated with a fluorine resin having a good mold releasing ability is provided such that a tip of the separating claw is brought in contact with an outer surface of a fixing roller or a fixing belt so as to separate a recording paper sheet from the fixing roller or the fixing belt.
As another aspect, in order to obtain an image in which an image surface is smooth all over the whole surface of the image and which has high glossiness without depending on an image density, well known is a gloss providing apparatus to transfer and fix a toner image composed of a transparent toner so as to provide gloss (refer to Patent Document 1: Japanese Unexamined Patent Publication No. 2002-341619).
Further, in order to make a photographic image area of a document to a gloss surface and a character image area to a gloss surface with semi-gloss, well known is an image forming apparatus which has a unit to develop with a transparent toner of semi-gloss and a transparent toner of gloss at an uppermost stream side in a conveying direction and develops a photographic image area with a glossy transparent toner and a whole area of images with a semi-glossy transparent toner so as to superimpose the semi-glossy transparent toner on a color toner layer (refer to Patent Document 2: Japanese Unexamined Patent Publication No. 2002-207334).
In the case where a separating claw is provided as described above, there are problems that since the tip of the separating claw is brought in contact with a surface of a fixing roller and the like, scratches are caused on a surface layer formed by fluorine resin so as to cover the surface of the fixing roller and the like, and such scratches are transferred onto an image. In this case, since a color image is required to be a gloss image, scratches tend to be exhibited appreciably on the color image.
For this reason, it is desirable to separate a recording paper sheet without employing a separating claw.